Path replacement of endpoint in Drive /upload operation

When we create a service **with endpoint** ([option#WithEndpoint](https://pkg.go.dev/google.golang.org/api/option#WithEndpoint)) using host and path, ie:
`https://myhost.com/connections/google/`

```go
service, err := drive.NewService(ctx, option.WithHTTPClient(client))
...
tokenSource := option.WithTokenSource(oauth2.StaticTokenSource(&oauth2.Token{AccessToken: *token}))
endpointOptions := option.WithEndpoint("https://myhost.com/connections/google/")
options := []option.ClientOption{
tokenSource,
endpointOptions,
}
```

it works well for operations like folder creation, as the URL will replaced by:
`https://myhost.com/connections/google/files`
Replaced by: https://github.com/googleapis/google-api-go-client/blob/007774894a48d24634ace7b9e09def4d61433f7f/drive/v3/drive-gen.go#L5520

But for media upload operations, like file creation, it is removing our endpoint path here:
https://github.com/googleapis/google-api-go-client/blob/007774894a48d24634ace7b9e09def4d61433f7f/drive/v3/drive-gen.go#L5522

Instead of calling this:
`https://myhost.com/connections/google/upload/drive/v3/files`
it's calling:
`https://myhost.com/upload/drive/v3/files`

The reason is the slash `/` at the beggining of the `relstr` during the `googleapi.ResolveRelative` func call.
This is the only operation that starts with slash, all others (like _files_, _comments_, etc) are appended to our custom endpoint.

Is it an issue or intentional?